Carotenoid Cleavage Dioxygenase 4 Catalyzes the Formation of Carotenoid-Derived Volatile β-Ionone during Tea (<i>Camellia sinensis</i>) Withering

The carotenoid-derived volatile β-ionone plays an important role in the formation of green and black tea flavors due to its low odor threshold, but its formation and the gene(s) involved in its biosynthesis during the tea withering process is(are) still unknown. In this study, we found that the content of β-ionone increased during the tea withering process catalyzed by an unknown enzyme(s). Correlation analysis of expression patterns of Camellia sinensis carotenoid cleavage dioxygenase genes (CsCCDs) and the β-ionone content during the withering period revealed CsCCD4 as the most promising candidate. The full-length CsCCD4 gene was amplified from C. sinensis, and the biochemical function of the recombinant CsCCD4 protein was studied after coexpression in Escherichia coli strains engineered to accumulate β-carotene. The recombinant protein was able to cleave a variety of carotenoids at the 9-10 and 9'-10' double bonds. Volatile β-ionone was detected as the main product by gas and liquid chromatography-mass spectrometry. The accumulation of β-ionone was consistent with the expression levels of CsCCD4 in different tissues and during the withering process. The CsCCD4 expression was induced by low temperature and mechanical damage stress but not by dehydration stress. The results demonstrate that CsCCD4 catalyzes the production of β-ionone in the tea plant and provide insight into its formation mechanism during the withering process.